The admission of hydraulic fluid to a servomotor of such a system is controlled, as is well known per se, by the incipient manual rotation of the steering shaft in one or the other direction, this incipient rotation causing the shifting of a main valve from an inoperative position into either of two working positions to pressurize the servomotor in the corresponding sense. It is generally desirable that the intervention of the servomotor be limited to the central part of the range of rotation of the steering shaft, the servomotor becoming inoperative as soon as the shaft approaches either limit of its range. Thus, it is known to provide at least one ancillary valve which, as soon as the shaft reaches either of two cut-out positions, short-circuits the servomotor by opening a bypass between its two feeder lines. The hydraulic fluid traversing the main valve then flows through the ancillary valve in series therewith, making it necessary for such valve to carry the entire circulation delivered by the supply pump.
For a smooth transition from power-assisted to purely manual steering, in the terminal phase of shaft rotation, the effective cross-section of the bypass valve or valves should be suitably dimensioned to depressurize the servomotor in a gradual manner, i.e., over a certain time interval. That interval, however, varies with the supply pressure and also with the viscosity of the hydraulic fluid (e.g. oil), being therefore dependent on temperature. If the bypass is too wide, the transition becomes rather abrupt; if it is too narrow, its flow resistance may be so large that the servomotor remains operative. Moreover, the presence of this bypass practically eliminates the ability of the servomotor to absorb the road shocks to which the dirigible wheels of the vehicle are subjected in their veering positions.